Machine tools exhibit dimensional positioning errors that are difficult to minimize. The primary contributors to these positioning errors are: (1) expansion and contraction of the machine structure and the workpiece (i.e., the part) because of thermal changes in the factory during machining, and (2) mechanical misalignments of and between individual axes of the machine. The accuracy of the machine is often so uncertain that post-machining inspection of the dimensions of the parts must be made using an independent measuring method. Such inspection requires special tools and skilled workers as well as significant factory space. It slows the production process. Failing inspection, parts must be reworked or scrapped. Post-production inspection, rework, and scrap are the result of poor design or manufacturing processes. The method of the present invention addresses the root cause for errors and, thereby, reduces the need for post-production inspection and the costs of poor quality.
A. Machine Error Control
National standards and “best practices” exist for determining and correcting NC machine geometry errors. (See ANSI/ASME B89.1.12M-1985, ANSI B89.6.2-1973, AMSE B.54-1992) These “best practices” constitute the currently accepted methods for achieving machine accuracy. We will discuss the standards and “best practices” briefly.
1. Thermally Controlled Environment
The machine is held at a constant temperature, e.g., 68° F., in an air-conditioned factory. Errors arising from temperature variations are reduced, but this method does not solve the thermal error problem entirely. Three main drawbacks are:                (i) The cost of controlling the environment is high and sometimes exceeds the cost of acquiring the machine.        (ii) Thermal effects induced by the machine itself (e.g. motor heat from driving under load, and spindle heating due to friction) still can cause machine distortion        (iii) Mechanical misalignment of axes remains uncorrected. Mechanical alignments change over time as the machine experiences normal and abnormal wear. They are essentially unpredictable, unavoidable, and difficult to control.        
2. Machine Calibration
Three-axis machines have 21 error parameters in addition to the errors introduced with the machine spindle. The errors are linearity in each axis (3), straightness in each axis (6), squareness between each axis pair (3), and pitch, yaw, and roll in and between each axis (9). Machine calibration measures some or all of these 21 error parameters, then makes physical or software adjustments to the parameters which are out of tolerance. Once each error is identified, quantified, and minimized, the combination of errors are summed using the root mean squares algorithm to gain an estimate for the machine's overall working tolerance. Machine calibration is inadequate for two reasons. First, the method requires extensive machine downtime to measure and to adjust the error parameters. The difficulty in the measurement and adjustment is compounded by the fact that thermal variation causes dimensional changes from shift to shift and day to day. Second, because of constant readjustment of the machine, the changes mean that the final set of data is not a single “snapshot” of the machine errors, but are a series of snapshots each of a different parameter, at a different time, as the machine changes. The root cause of inaccuracy is not fixed, but simply is accommodated between readjustments. Production is a compromise and drift occurs in the produced parts as the machine tool changes.
3. Linear Interferometry of Each Machine Axis
The X, Y, and Z axes of a machine are each equipped with a linear interferometer as an accurate positional encoder. The method allows real-time compensation for thermal growth and shrinkage, but is inadequate for at least three reasons. First, it cannot be applied to rotary axes. Second, it does not compensate for mechanical misalignments between axes. Third, it does not address the interaction between axes as thermal changes occur.
4. Volumetric Look-Up Table
This method accurately measures performance of the machine in a specified dimensional envelope. The accurate performance measurements are made using an independent, highly accurate measurement machine to determine the difference between the measured data and the commanded machine position. The collection of all such errors constitutes or can be used to generate an error map. A complete error map is used in two ways. First, the error map may be used as a look-up table to determine a simple position correction to the machine when in that vicinity. Second, polynomial equations can be calculated from the error map to interpolate error corrections over the entire measured envelope. The machine command for a position is adjusted with the polynomial equations. Look-up tables are inadequate primarily because the tables are valid for only one machine temperature. At other temperatures, the machine will be larger or smaller or have a slightly different geometry. There is no guarantee that a machine will behave isometrically and return to its original geometry as temperature changes occur. So, after a laborious data collection exercise leading to an empirical table or set of equations to adjust the position of the machine based upon its history of performance, the root cause(s) for inaccuracy will still continue to degrade the effectiveness of the error map. The error map is inherently inaccurate whenever the machine has changed. As the machine continues to wear and age, variations from the measured offsets of the original error map occur. As a result, errors in part construction may increase. Frequent recalibration is necessary to continue to have an accurate correct error map.
5. Combination of Methods
Certain combinations of these methods can be used to overcome weaknesses in the individual methods, but the net effect remains: (1) long downtime of the machine to measure its true position; (2) expensive testing; and (3) only temporary, corrective results. The root cause for the inaccuracies still remains. For instance, a combination of a thermally controlled environment with machine calibration can result in an accurate machine for a period of time. The cost of controlling the environment combined with the cost of machine downtime for checking and readjusting the machine can be expensive.
6. Thermal Compensation
The axes of the machine are equipped with thermal probes. The temperature measured by each probe is used to calculate independent from the other axes the theoretical expansion of that machine axis. The expansion factors are used to compensate the feedback to the controller, thus eliminating the expansion and contraction of the machine positioning capability. A newer but similar technique called “real time error correction” also uses thermal probes, but attempts to provide a 3D “error model” of the nonlinear thermal behavior of the machine structure. The error map reflects interdependence between axes, such as buckling or warping, caused by heating. Compensation is made with a complicated algorithm that is accurate only for the tested/measured envelope of variation and, then, only as the machine remains repeatable. This error model is established by gathering actual 3D machine position and corresponding temperature data over a range of temperatures, which can require significant machine downtime. It can also be difficult to place the machine in the desired thermal status. While the purpose of this technique is to avoid the costs associated with thermal control, thermal control is required to produce the error model. Thermal compensation follows the same concept as thermal control: modify the machine movement based on actual temperature measurements.
There are two main drawbacks to the thermal compensation method. First, thermal compensation requires periodic machine downtime to calibrate the sensors and the error model. Second, thermal compensation focusing on the machine does not correct for the expansion of the part or tooling fixtures. If it were possible to eliminate all positioning errors of the machine and perfectly to adjust the machine for temperature, the part could still be made out of tolerance because of the temperature effects on the part. Thermal compensation attempts to compensate for the part expansion indirectly by compensating for the machine errors caused by temperature changes. The correlation between the machine errors and the total error, however, is only a partial solution.
In U.S. Pat. No. 4,621,926, Merry, et al. describe an interferometer system for controlling non-rectilinear movement of an object. The system uses three, one-dimensional tracking laser interferometers rigidly mounted in a tracker head to track a single retroreflector mounted on the machine tool end effector. The Merry system is difficult to retrofit to an existing control system for a machine, because its laser feedback is designed to replace the conventional machine controller.
In the system of the present invention, the laser tracker operates independently from the machine controller to provide positional feedback information to the controller in trickle-fed Media blocks. [By “trickle feed” we mean that motion control information is provided (downloaded) to the machine controller a little bit at a time (in single NC Media blocks, for example) rather than as a complete program.] Our much larger working envelope (ten times larger than Merry) uniquely makes our system applicable to the manufacture and assembly of large aerospace structure, like wings, and our system design allows implementation readily on a large variety of existing machine controllers.
Merry determines the location of the retroreflector using trilateration. During set up and calibration, the machine moves in a straight line at constant speed along one independent axis for the system to establish a frame of reference for the end effector and to provide coordinate data to connect the laser interferometric position measurements with the end effector motion. Each interferometer is a one-dimensional (single axis) measurement system which generates a signal proportional to the distance of the retroreflector from the interferometer. With three output signals, the Merry control system uses trilateration to calculate the location of the end effector, compares this location with the desired location based upon a stored, predetermined path for motion of the end effector (i.e., the NC program), and actuates the tool's motive assembly to move the end effector to the next desired location. Laser trilateration has not been adopted in industry because of its cost, instability, setup geometry requirements, and natural inaccuracy. Trilateration works best if the three interferometers are widely spaced, but the retroreflector is essentially a one-axis target. To track the target, the interferometers must be close together which introduces significant interpolation or calculation errors. Futhermore, trilateration actually requires four interferometers to determine absolute, true position.
Merry's system replaces the standard machine controller with laser interferometric position measurement actually and directly to control of the tool. By overriding the machine controller, control of the machine might be lost, for example, if chips obscure the laser beam. For high value parts, the risk of loss of control is unacceptable. The Merry system, accordingly, has not been implemented for practical use in industry because of the problems it poses.
In a preferred embodiment of the present invention, static optical machine control (SOMaC) is able to adjust the machine media to accommodate translations, rotations, or both of the machine, part, or both. SOMaC does so by measuring the position of the part and the machine and scaling for changes from the original reference location and orientation of the part and machine. SOMaC also can adjust (scale) the machine media to accommodate changes in the part, machine, or both arising from changes in factory temperature, temperature of the part, temperature of the machine, and other physical changes in the factory environment.
The SOMaC system of the present invention provides fail-safe machine control because it continues to use the machine tool's conventional encoders, but augments the true position accuracy in static operation by providing “on-the-fly” inspection feedback through optical measurement of the true position. Our system corrects for the machine positioning errors with trickle feed instructions when the machine is at rest and ready for its next machining operation.
The Merry system cannot determine the location of the workpiece in relationship to the machine using the three interferometers alone. SOMaC is able to locate the machine relative to the workpiece using the single interferometer. Knowing this reference, SOMaC can provide delta correction commands to the machine controller after measuring the true position of the machine's end effector to enhance the machine's accuracy.
B. Laser Trackers
Real-time 3D optical measurement systems (e.g. laser trackers) are state-of-the-art measurement systems that obtain large quantities of accurate 3D data quickly. These optical measurement systems typically include an absolute ranging capability and a motorized angle steering head to steer the laser beam. The steering is controlled by a feedback system that continually drives the laser beam to follow (“track”) the retroreflector. The laser beam is directed from the laser tracker head into a retroreflective target which is mounted on the machine end effector. The return beam allows the tracking head to determine both the distance and the direction (i.e., the horizontal and vertical angles) to the retroreflector. These three measurements (range, horizontal angle, vertical angle) establish a spherical coordinate system which can easily be transformed into the Cartesian coordinate system.
Laser tracking systems have the following characteristics:                (1) Accurate 3D measurement of about 10 part per million (ppm) volumetric accuracy (0.1 mm in a 10 meter volume);        (2) Real-time measurement collection and transmission;        (3) Data rates, in excess of 500 3D measurements per second (and typically as high as 1000 measurements per second);        (4) Simple calibration;        (5) Virtually immune to errors caused by changes in air temperature and pressure when using a high quality compensator (refractometer); and        (6) Large measurement volume using a retroreflective target, typically a partial sphere up to 100 feet in diameter.        
Absolute ranging tracking interferometers can reaquire a target that has been temporarily blocked. Absolute ranging tracking interferometers are highly desirable in manufacturing operations, because movement of the machines, parts, and operators in the factory can lead to beam breaks. We prefer to use absolute ranging tracking interferometers, but many of our applications can also use the interferometer systems that are less tolerant of beam breaks.
Laser trackers have been used in many applications such as measuring the digital contour of aircraft or automobiles, tooling inspections, and NC machine accuracy testing. The present invention currently uses laser trackers, but other optical or non-contact measurement systems can be substituted for these systems to provide the positional feedback for the system.
In the aerospace industry, gantry or post-mill drilling machines range in size up to 70 meters long. The largest of these machines have working volumes in excess of 700 cubic meters. The positioning tolerance requirements for these machines are typically less than 0.20 mm. Attaining 0.50 mm positioning uncertainty within a 100 cubic meter volume is difficult. To standardize the uncertainty statement for NC machines, it is common to state the uncertainty of the machine in parts per million (ppm). The uncertainty, multiplied by one million then divided by the longest diagonal distance in the machine volume is the capability in terms of parts per million (ppm). For example, a typical machine with a 0.5 mm positioning capability and 15 meter diagonal length would yield a capability of 33 ppm. Large volume drilling machine capability below 30 ppm is difficult to achieve. As manufacturers strive to improve part quality and reduce assembly costs, the demand for more accurate hole drilling has increased. In aerospace manufacturing, these tighter tolerances can be as small as 0.10 mm over a 15 meter diagonal, which yields a standardized requirement of 6.7 ppm. Such tolerances exceed the capability of most machines.